This invention relates to a clay coating composition for use in pressure-sensitive duplication systems and the method for making this composition. The invention further relates to a modified clay-coated sheet for use in pressure-sensitive duplication systems.
The pressure-sensitive duplication systems of the prior art are generally based on the color-forming reaction between an electron-donating colorless compound and an electron-accepting solid substance. Examples of this type of system are described in U.S. Pat No. 2,712,507 of Green and U.S. Pat. No. 2,730,456 of Green, et al.
In the prior art duplication systems, a number of ways to facilitate the contact of the color-forming reactants have been devised. These systems include (1) a combination of an upper sheet having on its lower surface a layer of microcapsules containing an electron-donating colorless compound (referred to as a color-developing substance) and a lower sheet having on its upper surface a layer of an electron-accepting solid material (referred to as a color-forming substance); (2) a three-sheet combination wherein the intermediate sheet is coated on one surface with the color-developing substance and on the other surface with the color-forming substance; and (3) a single sheet being coated on the same surface with both the color-forming and color-developing substances.
Illustrative of the electron-donating colorless compounds used as color-developing substances in pressure-sensitive duplication systems are malachite green lactone, benzoyl leuco methylene blue, crystal violet lactone, 3-methyl-2, 2'spirobi (benzol [f] chromene), 3-dialkylamino-7-dialkylamino-flourans, alkyl- or aryl- substituted spiro-dinaphthopyrans and alkyl-or aryl-substituted spiro-dibenzopyrans. Electron-accepting solid substances suitable for use in pressure-sensitive duplication systems as a colorforming substance include active clay, acid clay, bentonite, kaolin, talc, aluminum silicate, aluminum citrate, zinc oxide, titanium oxide, aluminum oxide, arsenic oxide, zinc chloride, aluminum chloride, stannous or stannic chloride, and antimony chloride (see, for example, U.S. Pat. No. 3,442,908 of Orita, et. al.).
Clay compositions are also used generally in the paper industry as fillers and coating compositions for ordinary paper stock. Rowland, U.S. Pat. No. 2,307,239, discloses such a general purpose clay composition which is made by forming a dry mixture of kaolin and a metal salt such as sodium chloride, potassium chloride, aluminum chloride, etc., followed by heating in the range of 785.degree. to 965.degree. C. for between one and five hours.
While the pressure-sensitive duplication systems of the prior art generally exhibit good image formation, the prior art has failed to provide a system which will remain colorfast upon extended exposure to light.